why are consanguiuneous marraiges/matings NOT recommended?

Answer 1

First, society has deemed it immoral to marry someone who is closely related to you.
Anyway, the scientific basis against consanguineous marriages: there are ceratin diseases that are relatively uncommon and found only in certain families. You might call them the recessive traits/diseases. If only one allele is affected by the recessive gene, whoever has it becomes a carrier. Meaning, he has the gene but does not manifest the trait. If a person who is heterozygous for the trait (only one allele affected out of the two) marries another who is normal (no gene affected), the results would be children without the recessive gene, and some who are carriers (in a ratio of half and half).
Consider x as the recessive gene:
X x
X XX Xx
X XX Xx
See in the above pairing that half of the progeny are normal, and half have the recessive gene but not the trait.

If one of the parents is homozygous for the trait, she will manifest the traits, and all of her children will be carriers if the other parent is homozygous dominant.

If the progeny marry and have children together, they increase the likelihood of producing children who will manifest the abnormal trait. For example, if two of the children above who are carriers marry and have children:
X x
X XX Xx
x Xx xx
One-fourth of the children will be homozygous dominant, half will be carriers, and one-fourth will manifest the trait. Now, imagine these people marrying other family members, perpetuating the negative trait in their own family.

Answer 2

Because it's bad to "mix blood."

When you mate with family members, the genetic pool becomes too crowded and the offspring don't have the benefit of selecting different genes.

That's why children from related parents may be born with more genetic problems than children with unrelated parents.

Evolutionarily speaking, it is always best to select from a wider set of genes because then the offspring has a better chance to choose the best.